ice sheets

Ian Joughin/The Associated Press

This handout image provided by Ian Joughin, shows surface melt water rushing along the surface of the Greeland Ice Sheet through a supra-glacial stream channel on July 4, 2012, southwest of Ilulissat, Greenland. Polar ice sheets are now melting three times faster than in the 1990s, but so far that's added just less than half an inch to already rising global sea levels, a new giant scientific study says.

A study believed to be the most comprehensive ever done of the planet’s ice sheets shows that, overall, they are melting faster and faster.

And as world leaders meet in Doha, Qatar, to discuss responses to climate change, the paper published Thursday in the journal Science shows the loss of ice from Greenland and the Antarctic is making an ever-greater contribution to rising sea levels.

“It’s an observation with sufficient certainty to tell people the changes in the polar ice sheets are very much in line with what we expect those changes in climate to produce,” said co-author Andrew Shepherd of the University of Leeds.

Shepherd was one of an international team of 47 scientists who combined a wide array of data from 10 satellite missions and other sources to provide the clearest picture yet of what has been happening with the massive sheets of ancient ice that help anchor Earth’s climate at both ends.

Previous attempts to assess those sheets — the paper lists 29 of them since 1998 — have resulted in widely varying estimates. Combining the data sets from those previous studies, including important information from Canada’s Radarsat, has yielded the best measurement yet, the report says.

Overall, the sheets have lost about 4,250 gigatonnes of ice since 1992, enough to raise the average sea level around the globe by 11 millimetres.

That may not sound like much, but Erik Ivins of California’s Jet Propulsion Laboratory says it’s enough to matter.

“When you have 11 millimetres of increased sea level, if you compute the amount of mass that’s capable of coming on shore during storm surge, it’s a lot of mass,” he said. “Small changes in sea levels in certain places mean very big changes in the kind of protection of infrastructure you need to have in place.”

The refined measurements also allowed the scientists to get more accurate regional readings.

“Antarctica appears to be more or less constant, although it appears for the last decade we appear to see about a 50 per cent increase ice loss rate,” Ivins said. “Greenland is losing mass at about five times the rate today as it was in the early 1990s.”

That means melted ice is making an increasingly large contribution to overall sea level rise. Responsible for about 20 per cent of total sea level rise in the 1990s, the caps now make up to 40 per cent of the annual rise.

“In the last 10 years the contribution has gone up significantly compared with the previous decade,” said Michiel van den Broeke of Utrect University in the Netherlands.

The rest of the sea level rise is attributed to the increasing volume of seawater as it warms up.

The authors warn their study can’t be used to make hard predictions about what’s going to happen to the ice caps in the future.

“We thought we understood ice sheets, but as we observed them closer and closer we found we didn’t understand them very well,” Ivins said. “Projecting into the future may be as uncertain as it was four years ago.”

Still, Thursday’s study does provide a warning, said Ian Joughin of the Lamont-Doherty Earth Observatory in California.

“We can’t extrapolate, but we can see the trend is toward increases.”

The study comes on the heels of another report released earlier this week in Doha.

The United Nations weather agency said sea ice reached “a new record low” in the area around the North Pole this year and that the loss from March to September was a staggering 11.83 million square kilometres an area bigger than the United States.